1. Field of the Invention
This invention relates generally to a process for making a high gate current HEMT diode and, more particularly, to a process for forming a large area, high gate current HEMT diode using a citric acid etch.
2. Discussion of the Background
Various methods have been proposed to manufacture high electron mobility transistor (HEMT) devices. These devices are typically formed as integrated circuits having both diodes and transistors. While such methods have been used to produce HEMT devices, new uses for the devices which have more stringent requirements, as well as a continual reduction in design size require additional processes in order to achieve the desired arrangements.
Currently, there is a need to produce a HEMT diode that has a smaller total area, but which has a large diode periphery. The periphery is determinative of the current handling capability of the diode. Typically, the HEMT diodes have had the same size gates as the HEMT transistors in the same integrated circuit. For high frequency requirements, the gate length of the transistor has to be small (i.e. submicron dimension). However, such a small gate cross-sectional area is inappropriate for high current diode applications. The large current levels can often cause electromigration of the gate metal when the gates are of this submicron dimension. Previously, in order to handle this situation, the diodes were formed with several fingers to avoid current crowding and electromigration. However, in order to have large periphery diodes, many fingers have to be arranged in parallel which results in a diode occupying a large surface area of the substrate.
Furthermore, it has not been possible to fabricate large area gate HEMT diodes along with small gate HEMT transistors because the sub-micron gate etchant cannot uniformly etch the larger gates. Accordingly, there is a need for a process of making HEMT IC devices to produce diodes having a large periphery but a small area that can handle high gate currents without crowding or electromigration problems.
In general, a previous process for forming a large area diode is seen in FIG. 1A-1F. In FIG. 1A, a starting material is a semiconductor wafer of a heterostructure field effect transistor. In FIG. 1B, the wafer is spin coated with a photoresist so that a thin layer (1,000 nm) of photoresist is left on the wafer. Many different photoresists may be used, such as photoresist 1813 manufactured by Shipley Corporation. The photoresist is patterned and the pattern is developed using an aqueous chemical developer as shown in FIG. 1C. The wafer is dipped into a solution which etches the exposed semiconductor material, as shown in FIG. 1D. A metal is evaporated onto the wafer as shown in FIG. 1E. The metal may be a laminate of titanium/gold, molybdenum/gold, platinum/titanium/gold or other metals, as is known. Typical titanium and platinum thicknesses are 20-40 nm, while typical gold thickness is 500-700 nm. The resultant wafer is then put into a solvent such as acetone in order to remove the remaining photoresist and metal which is on the photoresist. The only remaining metal is that which is in the etched area of the wafer. The metal in the etched area forms the gate for a large area diode.
When using this process to form HEMT IC devices, there has been a problem that the etchant is not uniform and that for large area diodes, the etchant often produces a trench along the edge of such a diode or does not etch uniformly over the large area. That is, the etchant removes too much material at the base of a vertical section of the diode.
The prior art has also taught the use of a different type of etchant using citric acid, as described in the article xe2x80x9cHighly Selective Citric Buffer Etch-Stop Process for the Manufacture of Very Uniform GaAs/AlGaAs FETsxe2x80x9d by Schmukler, et al, 1993 IEEE GaAs IC Symposium Tech. Digest, page 325-328. However, this type of etchant has not been utilized in regard to the formation of large area HEMT diodes.
Accordingly, one object of the invention is to provide a simple and inexpensive process for forming an HEMT IC device.
Another object of this invention is to provide a method for forming a large area, high gate current handling diode on a HEMT heterostructure.
Another object of this invention is to provide a method for forming an HEMT IC device with different size gates for the diodes and transistors using the same etchant.
A still further object of this invention is to provide a method for forming a large area diode having a large diode periphery and an HEMT IC device utilizing a citric acid etchant.
Briefly, these and other objects of this invention are achieved by providing a different type of etchant in the process for forming an HEMT device where the etchant includes potassium citrate, citric acid and hydrogen peroxide.